i. Physical Layer
- specifies the physical interface between the node (computer, terminal) and the link that connected to X.25 network.
- specifies a protocol called X.21
ii. Frame Layer
- provides a reliable data transfer process through data link control which used link access procedure, balanced (LAPB) protocol.
- There are 3 categories of frame involved in the LAPB frame format: I-Frames – encapsulate PLP packets from the network layer and before being passed to the physical
layer S-Frames – flow and error control in the frame layer U-Frames- used to set up and disconnect the links between a DTE and a DCE.
- In the frame layer, communication between a DTE and a DCE involves three phases:
1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect
iii. Packet Layer Protocol (PLP)
Four types of PLP packet fields exist:
• General Format Identifier (GFI)—Identifies packet parameters, such as whether the packet carries
user data or control information, what kind of windowing is being used, and whether delivery
confirmation is required.
• Logical Channel Identifier (LCI)—Identifies the virtual circuit across the local DTE/DCE
Interface
• Packet Type Identifier (PTI)—Identifies the packet as one of 17 different PLP packet types.
• User Data—Contains encapsulated upper-layer information. This field is present only in data
packets. Otherwise, additional fields containing control information are added.
X.25 protocol specifies three layers:
i. Physical Layer
- specifies the physical interface between the node (computer, terminal) and the link that connected to X.25 network.
- specifies a protocol called X.21
ii. Frame Layer
- provides a reliable data transfer process through data link control which used link access procedure, balanced (LAPB) protocol.
- There are 3 categories of frame involved in the LAPB frame format:
I-Frames – encapsulate PLP packets from the network layer and before being passed to the physical
layer
S-Frames – flow and error control in the frame layer
U-Frames- used to set up and disconnect the links between a DTE and a DCE.
- In the frame layer, communication between a DTE and a DCE involves three phases:
1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect
iii. Packet Layer Protocol (PLP)
Four types of PLP packet fields exist:
• General Format Identifier (GFI)—Identifies packet parameters, such as whether the packet carries
user data or control information, what kind of windowing is being used, and whether delivery
confirmation is required.
• Logical Channel Identifier (LCI)—Identifies the virtual circuit across the local DTE/DCE
Interface
• Packet Type Identifier (PTI)—Identifies the packet as one of 17 different PLP packet types.
• User Data—Contains encapsulated upper-layer information. This field is present only in data
packets. Otherwise, additional fields containing control information are added.
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